The present invention relates to a process for orienting rod-like liquid crystal molecules. The invention also relates to an optical compensatory sheet comprising a transparent support, an orientation layer and an optically anisotropic layer formed from rod-like liquid crystal molecules in this order. The invention further relates to a polarizing plate using the oriented rod-like liquid crystal molecules.
A liquid crystal display comprises a liquid crystal cell, a polarizing element and an optical compensatory sheet (phase retarder). In a liquid crystal display of transmission type, two polarizing elements are arranged on both sides of the liquid crystal cell and one or two optical compensatory sheets (phase retarders) are arranged between the liquid crystal cell and the polarizing elements. On the other hand, a liquid crystal display of reflection type comprises a reflection plate, a liquid crystal cell, an optical compensatory sheet and a polarizing element in this order.
The liquid crystal cell comprises a pair of substrates, rod-like liquid crystal molecules and an electrode layer. The rod-like liquid crystal molecules are provided between the substrates. The electrode layer has a function of applying a voltage to the rod-like liquid crystal molecules. Each of the substrates has an orientation layer, which has a function of aligning the rod-like liquid crystal molecules. The alignment of the rod-like liquid crystal molecules is determined according to a display mode of the liquid crystal cell. Various display modes of the liquid crystal cell have been proposed. Examples of the mode for transmission type include TN (Twisted Nematic) mode, IPS (In-Plane Switching) mode. FLC (Ferroelectric Liquid Crystal) mode, OCB (optically Compensatory Bend) mode, STN (Super Twisted Nematic) mode, VA (Vertically Aligned) mode and ECB (Electrically Controlled Birefringence) mode. Examples of the mode for reflection type include TN mode, HAN (Hybrid Aligned Nematic) mode and GH (Guest-Host) mode.
The optical compensatory sheet has functions of removing undesired color from a displayed image and enlarging a viewing angle. As the optical compensatory sheet, a stretched birefringent film has been conventionally used.
Recently, an optical compensatory sheet comprising an optically anisotropic layer on a transparent support has been proposed in place of the stretched birefringent film. The optically anisotropic layer is formed from liquid crystal molecules. Since the liquid crystal molecules have various alignment forms, an optical compensatory sheet obtained by using the liquid crystal molecules has specific optical characteristics that cannot be obtained by the conventional stretched birefringent film.
The optical characteristics of the optical compensatory sheet are determined according to optical characteristics (i.e., displaying mode) of the liquid crystal cell. Various optical compensatory sheets properly applied for various display modes can be produced by using liquid crystal molecules. As the liquid crystal molecules for optical compensatory sheet, rod-like or discotic liquid crystal molecules are generally used.
Various optical compensatory sheets using liquid crystal molecules according to various display modes have been disclosed. For example, an optical compensatory sheet for TN mode is disclosed in Japanese Patent Provisional Publication No. 6(1994)-214116, U.S. Pat. Nos. 5,583,679, 5,646,703 and German Patent Publication No. 3,911,620A1. An optical compensatory sheet for IPS or FLC mode is disclosed in Japanese Patent Provisional Publication No. 10(1998)-54982, and a sheet for OCB or HAN mode is disclosed in U.S. Pat. No. 5,805,253 and International Patent No. WO96/37804. Further, a compensatory sheet for STN mode is disclosed in Japanese Patent Provisional Publication No. 9(1997)-26572, and one for VA mode is disclosed in Japanese Patent No. 2,866,372.
In an optical compensatory sheet having an optically anisotropic layer formed from rod-like liquid crystal molecules, an average direction of lines obtained by projecting long axes of the rod-like liquid crystal molecules on the support corresponds to a slow axis of the compensatory sheet. The average direction generally further corresponds to a rubbing direction of an orientation layer. The optical compensatory sheet is practically produced in the form of a roll. It is the easiest way to conduct a rubbing treatment along a longitudinal direction of the rolled sheet. Accordingly, the most easily produced optical compensatory sheet having an optically anisotropic layer formed from rod-like liquid crystal molecules has a slow axis along the longitudinal direction.
In a polarizing element of stretched polymer film, the transparent axis is perpendicular to the stretching direction. The polarizing element is also practically produced in the form of a roll. It is the easiest way to stretch the film along the longitudinal direction of the roll. Accordingly, the most easily produced polarizing element has a transparent axis perpendicular to the longitudinal direction (i.e., the transparent axis of that polarizing element is parallel to the lateral direction).
Where the rolled compensatory sheet is laminated with the rolled polarizing element, it is the easiest way to arrange the slow axis of the sheet essentially perpendicularly to the transparent axis of the element.
However, the slow axis of the compensatory sheet is preferably arranged essentially parallel to the transparent axis of the polarizing element in some display modes of liquid crystal cell.
The rod-like liquid crystal molecules must be aligned so that their long axes may be perpendicular to the rubbing direction of the orientation layer to arrange the slow axis of the rolled sheet along the latitudinal direction. In the present specification, the words xe2x80x9cthe long axes of the rod-like liquid crystal molecules are perpendicular to the rubbing direction of the orientation layerxe2x80x9d means that an average direction of lines obtained by projecting the long axes of the rod-like liquid crystal molecules on the support is perpendicular to the rubbing direction. A new orientation layer having such function is required to align the rod-like liquid crystal molecules perpendicularly to the rubbing direction. A conventional orientation layer aligns the liquid crystal molecules parallel to the rubbing direction.
An object of the present invention is to align rod-like liquid crystal molecules perpendicularly to the rubbing direction.
Another object of the invention is to provide a rolled optical compensatory sheet having a lateral slow axis.
A further object of the invention is to provide a polarizing plate in which the slow axis of the optical compensatory sheet is easily placed essentially parallel to the transparent axis of the polarizing element.
The present invention provides a process for orienting rod-like liquid crystal molecules, comprising the steps of: coating a solution of a copolymer comprising repeating units represented by the formula (I) and repeating units represented by the formula (II) or (III) on a support to form a coated layer; rubbing a surface of the coated layer to form an orientation layer; coating a solution containing rod-like liquid crystal molecules on the orientation layer; and then drying the solution to orient the rod-like liquid crystal molecules so that an average direction of lines obtained by projecting long axes of the liquid crystal molecules on the support is essentially perpendicular to a rubbing direction of the orientation layer: 
in which each of R1 and R2 independently is hydrogen, a halogen atom or an alkyl group having 1 to 6 carbon atoms; M is proton, an alkali metal ion or an ammonium ion; L0 is a divalent linking group selected from the group consisting of xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94NHxe2x80x94, xe2x80x94SO2xe2x80x94, an alkylene group, an alkenylene group, an arylene group and a combination thereof; R0 is a hydrocarbon group having 10 to 100 carbon atoms or a fluorine atom-substituted hydrocarbon group having 1 to 100 carbon atoms; Cy is an alicyclic group, an aromatic group or a heterocyclic group; m is 10 to 99 mole %; and n is 1 to 90 mole %.
The invention also provides a rolled optical compensatory sheet comprising a transparent support, an orientation layer and an optically anisotropic layer formed from rod-like liquid crystal molecules in this order, wherein the orientation layer is made of a copolymer having repeating units represented by the formula (I) and repeating units represented by the formula (II) or (III), and the rod-like liquid crystal molecules are so aligned that an average direction of lines obtained by projecting long axes of the liquid crystal molecules on the support is essentially perpendicular to a rubbing direction of the orientation layer.
The invention further provides a rolled polarizing plate comprising an optically anisotropic layer formed from rod-like liquid crystal molecules, an orientation layer, a transparent support, a polarizing film and a transparent protective film in this order, said orientation layer being made of a copolymer comprising repeating units represented by the formula (I) and repeating units represented by the formula (II) or (III), said rod-like liquid crystal molecules being so aligned that an average inclined angle between long axes of the liquid crystal molecules and a surface of the support is less than 5xc2x0 and that an average direction of lines obtained by projecting long axes of the liquid crystal molecules on the support is essentially perpendicular to a rubbing direction of the orientation layer, and said average direction being essentially parallel to a transparent axis of the polarizing film:
In the present specification, the term xe2x80x9cessentially perpendicularxe2x80x9d and xe2x80x9cessentially parallelxe2x80x9d means that the angle between the noticed directions is within the range of 90xc2x0xc2x15xc2x0 and 0xc2x0xc2x15xc2x0, respectively. The allowance of the angle is preferably less than xc2x14xc2x0, more preferably less than xc2x13xc2x0, further preferably less than xc2x12xc2x0, and most preferably less than xc2x11xc2x0.
The applicants have succeeded in aligning rod-like liquid crystal molecules uniformly and essentially perpendicularly to the rubbing direction by using an orientation layer containing a specific (meth)acrylic copolymer. The copolymer has a side chain containing a hydrocarbon group of 10 to 100 carbon atoms, a side chain containing a fluorine atom-substituted hydrocarbon group or a cyclic structure directly attached to a main chain. In an optical compensatory sheet having the orientation layer made of that copolymer, the rod-like liquid crystal molecules are aligned essentially perpendicularly to the rubbing direction. Accordingly, a rolled optical compensatory sheet having a slow axis perpendicular to the longitudinal direction (i.e., along the lateral direction) can be easily produced.
Further, a rolled polarizing element having a transparent axis perpendicular to the longitudinal direction (i.e., along the lateral direction) can be also easily produced. Therefore, a polarizing plate in which the slow axis of the compensatory sheet is essentially parallel to the transparent axis of the polarizing film can be easily produced by laminating the rolled compensatory sheet and the rolled polarizing element of the invention with their rolled forms maintained.
According to the invention, it is easy to arrange the slow axis of the compensatory sheet essentially parallel to the transparent axis of the polarizing film.